1. Field of the Invention
The present invention relates to a planetary gear apparatus. More particularly, the present invention relates to a planetary gear apparatus which can prevent a distance between input and output planetary gears coupled to a connecting shaft provided with a resilient support member from being reduced by more than a predetermined distance, thereby preventing loose feel caused by an axial movement of the planetary gears by a distance which the resilient support member is extended or compressed.
2. Description of the Prior Art
FIG. 1 illustrates a configuration of an active front wheel steering apparatus of a vehicle according to the related art. FIG. 2 is a sectional view schematically illustrating a gear ratio converting unit of an active front wheel steering apparatus of a vehicle according to the related art.
As illustrated in FIG. 1, the active front wheel steering apparatus according to the related art includes: a steering wheel 101; a steering shaft 103 connected to the steering wheel 101; a steering column 105 surrounding the steering shaft 103; an input angle sensor 107 provided at a lower end of the steering column 105; a gear ratio converting unit 109 provided below the input angle sensor 107; a first motor 111 for driving the gear ratio converting unit 109; a first motor position sensor 113 provided to the first motor 111; an output angle sensor 115 provided below the gear ratio converting unit 109; a vehicle speed sensor 117 for sensing a speed of a vehicle; a rack-pinion mechanism 121 provided below the gear ratio converting unit 109 and converting a rotary motion of the steering shaft 103 into a linear motion of a rack bar 119; a second motor 123 for supplying auxiliary steering power to the rack bar 119; and an Electronic Control Unit (ECU) 125 for receiving various types of signals from the input angle sensor 107, the first motor position sensor 113, and the output angle sensor 115 and controlling the first motor 111.
As illustrated in FIG. 2, the gear ratio converting unit according to the related art includes: an input shaft 210; a first sun gear 220 connected to a lower end of the input shaft 210; first planetary gears 270 externally meshed with the first sun gear 220; second planetary gears 280 connected with the first planetary gears 270 through planetary gear shafts 275; a second sun gear 230 externally meshed with the second planetary gears 280; an output shaft 240 connected with the second sun gear 230; a worm wheel 250; and a worm 260.
An operation principle of the active front wheel steering apparatus of the vehicle and the gear ratio converting unit according to the related art is as follows.
When a driver operates the steering wheel 101, the input shaft 210 rotates, and the first sun gear 220 provided at the lower end of the input shaft 220 rotates in conjunction with the input shaft 220.
In this case, the first planetary gears 270 externally meshed with the first sun gear 220 rotate, and the second planetary gears 280 connected with the first planetary gears 270 through the planetary gear shaft 275 rotate.
Finally, the second sun gear 230 externally meshed with the second planetary gears 280 rotates, and accordingly, the output shaft 240 extending from the second sun gear 230 rotates, so that the steering force caused by the driver's operation for the steering wheel 101 is transmitted to the rack-pinion mechanism 121.
However, in the above-described processes, the first motor 111 does not operate. When the first motor 111 operates, there is a change as follows.
When electrical signals are generated by the vehicle speed sensor 117 for sensing a speed of a vehicle, the input and output angle sensors 107 and 115 for sensing a change in steering angle, and the first motor position sensor 113, respectively, and transferred to the electronic control unit 125, the electronic control unit 125 transfers the electrical signals to the first motor 111 to drive the first motor 111.
Furthermore, as the first motor 111 is driven through the above-described processes, the worm 260 connected to a motor shaft (not illustrated) rotates, the worm wheel 250 meshed with the worm 260 rotates, and finally, a carrier 290 integrally formed with the worm wheel 250 rotates, thereby adjusting a ratio of an output angle to an input angle.
When a vehicle travels at high speed, even if the input shaft 210, the first sun gear 220, and the first planetary gears 270 are rotated through a driver's unidirectional operation for the steering wheel 101, the electronic control unit 125 controls the first motor 111 such that the carrier 290 as well as the worm wheel 250 rotates in a direction of obstructing rotation of the second planetary gears 280 connected with the first planetary gears 270. As a result, the active front wheel steering apparatus of the vehicle makes the rotation angle of the output shaft 240 smaller than that of the input shaft 210 to help the driver safely drive the vehicle.
Meanwhile, when the vehicle travels at low speed for parking, the electronic control unit 125 controls the first motor 111 to rotate in a direction in which the rotation angle of the output shaft 240 becomes larger than that of the input shaft 210. As a result, the active front wheel steering apparatus of the vehicle helps the driver operate the steering wheel at a smaller angle.
However, the gear ratio converting unit according to the related art has a problem in that precise steering control is difficult due to a backlash caused by abrasion of gear teeth, and, in particular, the steering apparatus of the vehicle has problems in that noise caused by the backlash incurs unpleasant feelings during steering, and the noise becomes larger as abrasion of the gear teeth is developed.